1. Field of the Invention
The present invention relates to methods and apparatus for adding oxygen to and removing carbon dioxide from blood in vivo without involving the patient's lungs.
2. The Prior Art
Thousands of patients in hospitals each day are hypoexemic and/or have inadequate removal of carbon dioxide (CO.sub.2) from their circulating blood. These conditions of inadequate blood gas exchange, which include both inadequate blood oxygenation and inadequate removal of CO.sub.2, are commonly caused by varying degrees of respiratory inadequacy usually associated with acute lung illnesses such as pneumonitis, atelectasis, fluid in the lung or obstruction of pulmonary ventilation. Various heart and circulatory ailments such as heart disease and shock can adversely affect the flow of blood and thereby also reduce the rate of blood gas exchange.
Currently the most widely used methods of treating these types of blood gas exchange inadequacies involve increasing the flow of oxygen through the lungs by either increasing the oxygen concentration of the inspired gases or by mechanically ventilating the lungs. Both methods result in placing further pressure on the lungs, which may be diseased and unable to function at full capacity. In order to allow diseased or injured organs to heal it is generally best to allow those organs a period of rest followed by a gradual increase in activity. The current methods of treating inadequate blood gas exchange, however, force the diseased or damaged lungs to work even harder rather than allowing them a period of rest and recovery.
Various devices have been developed which are capable, at least for a limited period of time, of taking over the gas exchange function of the lungs. Many extracorporeal blood oxygenators are in common use and are employed most frequently during heart surgery. These devices are capable of providing blood oxygenation sufficient to carry the patient through the surgical procedure, at least when employed in conjunction with techniques, such as hypothermia, which reduce the body's demand for oxygen. These blood oxygenators include devices which bubble oxygen into the blood as the blood flows through the device. This is usually followed by a section of the device which defoams the blood to make it acceptable for reinjection into the patient.
Another group of extracorporeal oxygenators employ gas permeable membranes. These devices take many different shapes and configurations, however, the basic concept of operation is the same in all of these devices. Blood flows on one side of the gas permeable membranes while an an oxygen rich gas flows on the other side of the membrane. As The blood flows through the device, the oxygen travels across the gas permeable membrane and enters the blood. This allows oxygenation of the blood without actually introducing oxygen bubbles into the blood and without the corresponding need for an extensive defoaming apparatus.
Neither of the types of oxygenators discussed above are particularly adaptable for use outside the setting of a cardiopulmonary bypass procedure, and are thus typically designed only for short term extracorporeal use. As a result, these devices are not of use in the day-to-day treatment of the typical patient suffering from lung disease or circulatory deficiency.
If oxygen could be added to and carbon dioxide removed from the circulating blood simply, effectively and harmlessly so as not to molest or irritate the sick or failing lungs, thus permitting the lungs to temporarily rest and recover without adversely effecting needed blood gas exchange, a significant and potentially very widely applicable breakthrough in hospital care would have been realized.